U.S.-backed Iraqi forces launched a broad attack on the final districts held by the Islamic State (IS) militant group in west Mosul, military officials say.

The Joint Operations Command on May 27 said security forces are attacking "what remains of the unliberated areas" on the west bank of the Tigris River in the city, the last significant IS stronghold in Iraq.

The move came a day after the Iraqi Air Force dropped leaflets telling residents to flee crowded neighborhoods just north of the Old City, where most of the militant forces are clinging on as Iraqi forces encircle the district.

The statement did not specify if an attack was being carried out on the Old City itself.

The Associated Press quoted two Iraqi military officers as saying the advancement of government troops had been slowed by snipers and suicide bombers with clashes on Sunday described as being "sporadic." They gave no figures on casualties for either side.

Should Mosul fall to the Iraqi forces, it would represent a massive blow to the IS extremists. Mosul was the city where leader Abu Bakr al-Baghdadi declared his so-called "caliphate" nearly three years.

Iraqi military officials said they hoped to capture the Al-Nuri mosque, where Baghdadi announced the caliphate, in the next few days.

Residents in the Old City told Reuters news agency by phone that the situation there was becoming desperate.

"We're waiting for death at any moment, either by bombing or starving," one said. "Adults eat one meal a day, either flour or lentil soup."

United Nations and humanitarian officials estimate that 700,000 people -- one-third of the city's prewar population -- have already fled the fighting, with some 200,000 still thought to be trapped.

Iraqi forces backed by U.S. and coalition air support in October began the battle to liberate Mosul.

The eastern half of Mosul was retaken earlier this year, and troops are facing fierce IS resistance in more heavily populated west Mosul.

IS fighters captured Mosul in 2014 as they gained wide swathes of territory in Iraq and Syria in battles against government troops.

However, U.S.-led coalition forces have made major gains against the group, both in Mosul and in the Syrian city of Raqqa, the last major IS stronghold in that country.